Rotating-anode x-ray tube with a metal envelope and a frustoconical anode

ABSTRACT

An X-ray tube having an evacuated metal envelope in which an anode is mounted for rotation about an axis perpendicular to an electron beam produced by a cathode. The anode is cooled and is frustoconical in shape, the inclined surface of which intercepts the electron beam. In a preferred embodiment, the anode is in the form of a disc consisting of molybdenum or graphite surrounded by a tire of tungsten or an alloy of tungsten and is at ground potential while the cathode is at a potential which causes X-rays to be generated and is secured to the envelope by an insulator having a recess for receiving a plug connected to a source of potential sufficient to generate X-rays.

O United States Patent [151 3,646,380

Hart! [45] Feb. 29, 1972 [54] ROTATlNG-ANODE X-RAY TUBE WITH 2,594,564 12/1952 Kehrli ..313/56 A METAL ENVELOPE AND A FRUSTOCONICAL ANODE FOREIGN PATENTS OR APPLICATIONS 72] Inventor: Walter Hart], Stuffelring, Germany 222133 :31: [73] Assignee: U.S. Philips Corporation, New York, NY. 646,274 1950 Great Britain ..313/60 [22] Filed: 1969 Primary Examiner-Roy Lake [211 A M 851,169 Assistant Examiner-Darwin R. Hostetter Attorney-F rank R. Trifari Aug. i9, Germany An y tube having an evacuated mew] envelope i an anode is mounted for rotation about an axis perpendicular [52] US. Cl. ..3l3/60, 313/35, 3113/34;; to an electron beam produced by a cathode. The anode is H in CI il, cooled and is frustoconical in shape, the inclined surface of which intercepts the electron beam In a preferred embdi [58] Field Search 2 ag 8: ment, the anode is in the form of a disc consisting of molybdenum or graphite surrounded by a tire of tungsten or an alloy of tungsten and is at ground potential while the cathode is at a [56] Reierences cued potential which causes X-rays to be generated and is secured UNITED STATES PATENTS to the envelope by an insulator having a recess for receiving a plug connected to a source of potential sufficient to generate 2,427,203 9/1947 Essig ..313/60 X x 2,480,198 8/1949 Rogers 313/55 X 2,549,614 4/1951 Leighton ..313/60 X 4 Claims, 1 Drawing Figure PAIENTEBFEB 29 1912 1N VENTOR.

WALTER HARTL BY glwafi AGENT ROTATING-ANODE X-RAY TUBE WITH A METAL ENVELOPE AND A FRUSTOCONICAL ANODE The invention relates to a rotating-anode X-ray tube having a high-load capacity.

ln a rotating-anode X-ray tube of conventional construction, a plane disc is arranged for rotation about a shaft which extends substantially parallel to the direction of the electron beam. On one surface of the disc there is traced at least one focal-spot path the radius of which is limited to at most 80 percent of the radius of the disc, because it must be spaced from the edge of the disc for safety reasons. In operation the respective part of the disc is heated to a particularly high temperature; the thermal expansion causes pressure to be exerted on the areas lying outside the focal-spot path and a tensile force to be exerted on the areas of the disc lying within said path. These stresses cause the disc to become cracked in course of time.

Another reason why the temperature of the disc cannot be raised indefinitely is the impairment of the mechanical strength of the glass envelope at high temperatures. Finally, a rise in anode temperature increases the energy radiated by the anode towards the cathode with a consequent rise of the cathode temperature so that the likelihood of vaporization and hence defects in the tubes is increased. Hence, the maximum permissible continuous power does not exceed about 500 watts. Since in these tubes the rotating anode is supported by a bearing at one end only and the main mass (the disc anode) lies outside the bearing, in this construction with the usual mass distributions natural resonances occur at about 100 to 120 Hz. Consequently, limits are set to the speed and/or the diameter of the disc by the unbalance which varies in operation.

From the beginnings of X-ray technology rotating-anode X- ray tubes have been known (cf. German Pat. No. 636,695 and US. Pat. Nos. 1,192,706 and 1,621,926) in which the axis of the rotating anode is at right angles to the path of the electron beam. ln these tubes, the path of the focal spot lies on the curved surface of the cylindrical anode, so that the described uneven distribution of the mechanical stresses cannot occur in operation.

However, this construction has the disadvantage that the electron beam impinges laterally on the curved surface of the anode cylinder, i.e., at an angle of about 45 to the normal to this surface. This means that the position of the focal spot on the anode is subject to large fluctuations because of the grazing incidence of the electrons on the anode. Hence, the position of the focal spot depends among other things on the value of the applied voltage. Furthermore, the focal spot is shifted when the anode is not exactly circular in cross section, as may be the case even in accurately balanced anodes. Hence, these constructions have acquired no importance in practice so far.

According to the invention, in a rotating-anode Xray tube, in which the shaft about which the anode rotates is joumaled so as to extend at right angles to the direction of the electron beam, the above-described disadvantages are avoided in that the anode disc is shaped in the form of a frustum of a cone. The electron beam impinges in the direction of the normal to the axis of rotation and the X-rays produced are emitted substantially parallel to the axis of rotation.

The invention will be described more fully with reference to an embodiment given by way of example and illustrated in the drawing.

The drawing shows an X-ray tube having an grounded cylindrical metal envelope 1. Because a metal envelope is used instead of the usual glass envelope this tube need not be arranged in a protective casing, which generally is filled with oil. The lead coating necessary for reasons of protection against radiation can be directly applied to the metal envelope 1. The cathode 2, to which a high voltage is applied, is secured to one end of a ceramic insulator 3, the other end of which is joined to the front face of the metal envelope in a vacuumtight manner. For connection to the high voltage a conical highvoltage plug is entirely inserted in a matching recess in the ceramic insulator.

It should be noted that such an X-ray tube having a metal envelope and an insulator which is secured in the envelope and carries the cathodes is known from German Pat. No. 924,940.

In the lower part of the metal envelope there is arranged a disc anode 4, which is mounted on a shaft 6 which extends at right angles to the direction of the electron beam and is journaled in bearings 7 arranged in a copper block which is adapted to be cooled. With this positioning of the shah, only part of the thermal energy emitted from the focal spot reaches the cathode, so that the above-mentioned likelihood of evaporation of the cathode material is reduced. The rotating anode disc is shaped in the form of a frustum of a cone the curved surface of which is inclined at an angle of about 17 to 20 to the shaft and is surrounded along its edge by a tire 9 made of tungsten or tungsten-molybdenum or by a tape of a rhenium-tungsten-molybdenum ply material. Thus, the diameter of the path of the focal spot is substantially equally to the diameter of the disc, which in itself enables the short-time loading to exceed that obtainable in the conventional tubes in which the rotation shaft extends parallel to the electron path and in which the diameter of the path of the focal spot must be smaller than the disc diameter by about 20 percent. Between the anode and the cathode there is arranged a centrally apertured metal screen 16, which electrostatically screens the entire lower tube portion with the exception of the part of the focal-spot path on which the electron beam impinges from the cathode.

Because of the frustoconical shape which enables the X- rays produced to emerge laterally, i.e., in a direction parallel to the axis of rotation, the rotating anode can be arranged so relatively to the cathode that the electrons impinge on the tire 9 in the direction of the normal to this axis. Small deviations of the disc diameter from an exactly circular form have only a slight influence on the position of the focal spot; the value of the voltage set up between the anode and the cathode has no influence at all on this position. Consequently, the X-ray window 17 in the form of a beryllium plate can be arranged close to the focal spot. This enables diaphragms to be arranged at a distance of not more than 1 to 2 cm from the focal spot, so that the stray radiation emitted from metal components outside the focal spot is reduced to a minimum.

The rotating anode may be made of the usual material, i.e., molybdenum. However, because its mechanical strength is greatly increased by the tire 9 and because its temperature and voltage distribution is rotation symmetrical and symmetrical about the axis, it may alternatively be made of a material of lower mechanical strength but better thermal properties, such as graphite or glass carbon. For example, at the same temperature a graphite body radiates more heat than the molybdenum body as used for the conventional rotating anodes. In addition, graphite has a higher thermal load capacity than molybdenum so that the amount of heat radiated by the anode, which amount, as is known, increases as the fourth power of the temperature, is considerably greater.

Components 10 provided for securing the anode 4 and the shaft 6 are proportioned so that their thermal resistivity is sufficient to limit the amount of heat transferred by the hot anode disc to the bearings 7 so that the temperature of the bearings does not exceed 300 to 400 C. A cavity 11 provided in the copper block 8 prevents heat radiated by the anode disc to the inner surface of the block from reaching the bearings in the outer part of the block by the shortest possible path. in each cavity 11 a disc 12 of blackened copper is arranged which is secured to the shaft 6 and radiates part of the heat absorbed so that the ratio between the energy radiated and the energy transferred to the bearings is further increased. The cooling coils for water which extend close to the bearings contribute to prevent impermissible heating of the bearings.

Because the X-shaft is joumaled at both ends, the unbalance forces and the loads imposed on the bearings can be maintained small. Hence the mass of the rotating anode and/or its speed may be increased, and this involves a further increase of the thermal load capacity.

The rotating anode is driven by means of a disc-shaped rotor which is secured to the end of the shaft 6 remote from the window and forms part of an induction motor of a commutatorless direct-current motor arranged outside the tube envelope (not shown). In order to prevent the energy of the magnetic field from being converted within the metal envelope 1, in this area a ceramic disc 14 is arranged in a vacon ring 15 welded to the casing. The airgap, which in this case is defined by the ceramic disc and the evacuated space between the rotor t3 and the ceramic disc 14, in this embodiment may be considerably smaller than in a rotating-anode tube having an anode at which a high voltage is set up, so that with the same driving power a shorter starting time of the rotating anode or with the same starting time a smaller driving power is possible.

The anode disc can readily be balanced by means of bores in its plane faces, and this does not impair its high-voltage properties owing to the provision of the metal screen 16. During assembly the copper block 8 is preferably joined to the metal envelope by argon arc welding after the anode and the shaft have been placed in position.

In tubes of the type described the short-time load can be about doubled and a continuous load of about 6 kilowatts (as compared with 500 watts in the usual rotating anode tubes) is permissible. Hence, the tube is suitable not only for medical diagnostics but also for structure investigations.

What is claimed is:

1. An X-ray tube comprising a substantially cylindrical metal envelope, an electron-emitting element within the envelope for producing an electron beam in the axial direction of the envelope, a disc-shaped anode having a frustoconical circumferential surface positioned to intercept electrons, means to rotatably support the anode at right angles to the axis of the envelope including a shaft, a heat conducting metal support hermetically secured to the metal envelope, and bearing means positioned within said support for supporting said shaft.

2. An X-ray tube as claimed in claim I in which the support consists of copper and has channels for a circulating cooling fluid.

3. An X-ray tube as claimed in claim 1 in which the support has a cavity on each side opposite the anode between the anode and the bearing means in which rotatable blackened copper discs are secured to the shaft.

4. An X-ray tube as claimed in claim 1 in which the conductive support has a portion spaced from the wall of the envelope and a disc-shaped rotor member of an externally positioned induction motor rotatably supported by said shaft is positioned in the space between the conductive support and the inner wall of the envelope, and an insulating disc is provided in the wall of said envelope opposite said disc shaped rotor member.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Paten 3.646.380 mhll;

Inventor (s) WALTER HARTL' It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Change the priority date from "August 19, 1968" to -August 17, 1968-- Signed and sealed this11 day of July 1972.

(SEAL) Attest:

EDZIARD M.FLETCHER, JR. ROBERT GOTTSCHALK Attesting Officer Commissioner of Patents 

1. An X-ray tube comprising a substantially cylindrical metal envelope, an electron-emitting element within the envelope for producing an electron beam in the axial direction of the envelope, a disc-shaped anode having a frustoconical circumferential surface positioned to intercept electrons, means to rotatably support the anode at right angles to the axis of the envelope including a shaft, a heat conducting metal support hermetically secured to the metal envelope, and bearing meanS positioned within said support for supporting said shaft.
 2. An X-ray tube as claimed in claim 1 in which the support consists of copper and has channels for a circulating cooling fluid.
 3. An X-ray tube as claimed in claim 1 in which the support has a cavity on each side opposite the anode between the anode and the bearing means in which rotatable blackened copper discs are secured to the shaft.
 4. An X-ray tube as claimed in claim 1 in which the conductive support has a portion spaced from the wall of the envelope and a disc-shaped rotor member of an externally positioned induction motor rotatably supported by said shaft is positioned in the space between the conductive support and the inner wall of the envelope, and an insulating disc is provided in the wall of said envelope opposite said disc shaped rotor member. 